HVAC system controllers typically control devices which effect the environment in an enclosed space based on measurements of one or more environmental variables. For example, most HVAC systems include thermostats which sense the air temperature within a room and controllers which control the behavior of a heater or air conditioner to achieve a desired air temperature within the room based on the air temperature measurements.
However, factors other than air temperature significantly affect an occupant's comfort and security. For example, the ventilation rate (outdoor air flow rate) of an enclosed space is an important factor in indoor air quality. In particular, ASHRAE Standard 62-1989 states that the minimum outdoor flow rate should be 15 cfm/person.
To meet this standard, buildings have been designed so that, under typical circumstances, the actual flow rate within an enclosure will meet or exceed the recommended minimum outdoor flow rate. However, without a feedback controller to guarantee a 15 cfm/person ventilation rate, the actual outdoor flow rate for any given enclosure and situation may fall below the minimum recommended outdoor flow rate. In order to implement such a controller, the number of people in a room must be accurately measured. Based on the technology available in the prior art, it has proven difficult to accurately, inexpensively and unobtrusively make such measurements. Therefore, it is clearly desirable to provide a method for unobtrusively measuring the number of people occupying a room, and an apparatus for ventilating a room based on the actual number of occupants therein.
Moreover, when the number of occupants of a room changes abruptly, the extra heat and moisture created by the occupants should be compensated for by the HVAC system. One common way to compensate for load disturbances caused by time variable occupancy is to incorporate derivative action into the controller, so that changes in the number of occupants is quickly counteracted by adjustments made in the heat supplied. The drawback of this approach is that the controller will also be sensitive to noise in the measurement of the control variable. Additionally, disturbance rejection with feedback adversely effects the stability of a system. Therefore, it is desirable to provide a method for responding to time variable occupancy in such a way as to avoid instability and noise sensitivity.
Another approach to compensate for load disturbances caused by time variable occupancy is to use the average heat generated when the room is occupied as a feed forward term. However, the actual heat generated may vary greatly from the average heat generated because the number of occupants may differ significantly from the average. Also, such a system cannot determine when a room is occupied. Accordingly, it is desirable to provide a method for compensating for time variable occupancy using a feed forward term based on the actual number of occupants.
Factors which affect an occupant's comfort include air temperature, humidity, air velocity, clothing insulation, bodily heat production rate and mean radiant temperature. A system for providing adaptive control of HVAC systems based on these six parameters is disclosed in U.S. Pat. No. 5,170,935, issued Dec. 15, 1992 to Federspiel et al., the content of which is incorporated herein by reference. Unfortunately, it is not practically possible to directly measure some of these variables.
Consequently, prior art comfort control systems have been forced to make assumptions about the variables which are otherwise too difficult to measure. For example, one prior art comfort control system simply assumes that bodily heat production is known. Another prior art system assumes that bodily heat production is unknown but constant. These limiting assumptions reduce the accuracy of thermal comfort control methods. A more accurate estimate of bodily heat production could be made by measuring the pulse rate of all of the occupants of an enclosure via pulse rate sensors strapped to their wrists. Though this pulse-monitoring method may theoretically provide a superior thermal comfort control, it is clearly impractical for most real-world situations. Thus, it is desirable to provide a practical, relatively accurate apparatus and method for determining the amount of bodily heat generated by the occupants of an enclosed space.
Some building control systems are limited to addressing the physical comfort of occupants. However, it would also be desirable to provide a building control system that addressed the emotional comfort concerns of occupants as well. One factor affecting an occupant's emotional comfort is the amount of security a room or building provides. Specifically, people feel more secure when safeguards have been taken to inhibit and/or announce the presence of intruders. One's anxiety may be lessened, for example, if he or she does not have to search for a light switch upon first entering an unlit room. Likewise, one typically feels more secure when protected by an alarm system configured to detect and announce the presence of other occupants.
Consequently, security systems have been developed which detect the presence of humans. Prior art security systems detect humans based on sound, infrared radiation, or vision. Unfortunately, each of these systems has a flaw which may be exploited by intruders. For example, security systems based on sound can fail if the intruder is quiet. Infrared security systems can fail if the intruder is shielded. Vision systems can fail if the intruder is out of view or cannot be identified by the system. In addition, vision systems are also relatively expensive. Hence, it is clearly desirable to provide a method and apparatus for detecting the presence of an occupant which does not suffer the disadvantages of the present security systems.